Rail tank cars are used in bulk transportation to transfer petroleum products from one location to another. One skilled in the art also knows that specialized rail cars exist for the transport of asphalt or bitumen and specialized rail cars exist for diluent and similar light hydrocarbons such as gasoline (“diluent”). Asphalt is defined as residual crudes that have a specific gravity of about 1.04 and diluents are liquid hydrocarbons which have specific gravities of about 0.830 or less.
The approval for railroad tank car designs, materials, construction or alteration of tank cars are prescribed by the Executive Director Tank Car Safety, AAR (Association of American Railroads) and the Tank Car Committee. The US Department of Transportation in conjunction with the AAR have classified asphalt tank cars and diluent tank cars as DOT 111A100W1 (AAR 211A100W1). Both asphalt tank cars and diluent tank cars share common certification requirements for tank design and construction, type of steel, welding, repairs, documentation, mounting, coupler vertical restraint systems, pressure relief devices, and markings. Both these types of cars are classified as general purpose non-pressure tank cars (Department of Transportation Part 179 Specifications for Tank Cars: Subpart D—Specifications for Non-Pressure Tank Car Tanks: Classes DOT—111AW and 115AW).
However, asphalt cars and diluent tank cars have several significant differences:
Asphalt tank cars are smaller than diluent tank cars because the density of the asphalt is much higher. The density of liquid heated asphalt loaded into tank cars is typically in the range of 975 kg/cubic meters. The density of diluent typically loaded into tank cars at ambient temperature is in the range of 675 to 830 kg/cubic meters. Because tank cars are essentially limited to 286,000 lbs in maximum gross weight, and assuming that the asphalt car empty weighs 82,500 lbs, it is not logical to build an asphalt tank car with a shell capacity greater than about 25,500 gals (US), with a loaded capacity of 25,000 gals (at 98% of full load). The reason is once 25,000 gallons of 975 kg/cubic meters asphalt are placed in its tank the total gross rail load of the rail tank car is about 286,000 lbs.
Diluent rail cars are similar to general purpose gasoline, diesel and ethanol rail cars for reasons of interchangeability and fungability. The tank cars and products are interchangeable because the density and vapour pressure of these various products are similar. A 286,000 lb gasoline/diluent tank car can be designed to have a shell capacity of around 33,000 gallons (US), with a loaded capacity of 32,300 gals (at 98% loaded) and it has a vehicle weight of 72,000 lbs when empty. If the liquid diluent (or similar) product with a density of about 795 kg/cubic meters is placed in this rail car's tank then the rail car will reach its maximum allowable weight (286,000 lbs) when 98% loaded, at 32,300 gallons of diluent. The density of gasoline is about 720 kg/cubic meters, ethanol is 795 kg/cubic meters and diesel is about 830 kg/cubic meters. Diluents used for bitumen are also in this range.
This aspect, the railroad's weight restriction, is a major design factor when designing a tank car. This restriction is known by those skilled in the art.
Because asphalt does not flow efficiently at ambient temperatures, asphalt cars are insulated and, typically, externally heat traced. Heat tracing is almost always done with a heat exchange system of a steam coil affixed to the tank. Steam lines are attached to the tank car when it reaches an unloading facility. The steam in the coil transfers heat to the tank and that heat radiates into the asphalt increasing its fluidity. The steam may be supplied from either a stationary or a portable boiler. These cars are called coiled, insulated, non-pressurized tank cars. Because diluent does not “solidify” or become viscous at even low ambient temperatures, diluent cars are not heat traced. These diluent cars are called non-coiled, non-insulated, non-pressurized tank cars.
As steam lines add significant empty weight to the tank car, they are not installed unless required by the cargo for viscosity reduction purposes. This is known to one skilled in the art.
The tank on a diluent tank car requires a thicker head shell made of stronger steel, or a protective head shield to reduce the possibility of tank rupture if the car derails or is in collision. This requirement is set out by the DOT in section 179.169 (a) (b) (c) “Tank-head puncture resistance systems” of its Specifications for Tank Cars and Canada's National Transportation Act, Railway Act. Diluent tank cars also require the installation of a pressure relief valve (as set by the AAR Appendix A and DOT Section 179.15: Pressure relief devices), valve protection at the top of the tank (around the vacuum and safety pressure relief valves), and protective steel around the bottom discharge valve (DOT Specifications for Tank Cars—Section 179.200 and AAR Specifications for Tank Cars paragraphs E9.00 and E10.00).
This requirement for added safety features for diluent railcars is known by those skilled in the art.
Gasket materials must be selected from a group of materials that are effective with the commodity being transported and not negatively affected thereby. The US Department of Transportation (DOT) has made these aspects a requirement for diluent transportation by rail.
This requirement for special gaskets, seals, tubing, and valves suitable for use with diluent type solvents in transport is known by those skilled in the art.
Asphalt is loaded and unloaded into insulated tank cars at high temperature that the asphalt is liquefied to be pumped. The temperature of the asphalt during loading and unloading is typically between 300 deg F. and 325 deg F. (˜150 deg C.). However, diluent is placed in a tank car at ambient temperature, not elevated temperatures, to avoid dangerous or costly boiling/evaporation of any volatile hydrocarbon components.
These very different loading and unloading temperatures for diluent and asphalt or bitumen are known by those skilled in the art.
Because differences between the two types of tank cars are significant, tank car manufacturers do not build tank cars that meet the DOT requirements and combine asphalt and diluent transportation.